D&C 93

The Savior taught during His ministry -

"The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light." (New Testament | Matthew 6:22)

In modern day, He taught -

"...if your eye be single to my glory, your whole bodies shall be filled with light, and there shall be no darkness in you; and that body which is filled with light comprehendeth all things." (Doctrine and Covenants | Section 88:67)

The Lord also explained that -

"...truth is knowledge of things as they are, and as they were, and as they are to come..." (Doctrine and Covenants | Section 93:24)

From the combination of these two principles the Lord taught -

"The glory of God is intelligence, or, in other words, light and truth." (Doctrine and Covenants | Section 93:36)

The world may define intelligence or the state of being intelligent differently, but the Lord describes that when one lives the commandments and becomes more holy, the more he is filled with light. That light combined with the knowledge of all things is intelligence. Satan may have had knowledge but he possessed no light therefore unintelligent. One of my more favorite Presidents of the Church, Joseph F. Smith said -

"...inherited his intelligence from his Father. There is a difference between knowledge and pure intelligence. Satan possesses knowledge, far more than we have, but he has not intelligence, or he would render obedience to the principles of truth and light. . . . Pure intelligence comprises not only knowledge, but also the power to properly apply that knowledge." (The Way to Perfection, p. 231.)